Development of optical transmission module for access networks Hiroshi Ishizaki Takayuki Tanaka Hiroshi Okada Yoshinori Arai Alongside the spread of the Internet in recent years, high-speed data transmission services for subscriber networks (access networks) have taken root in a real way. This has been accompanied by a growth in fibre-optic communications networks. Fig. 1 illustrates an optical communications network. The core D (Dense Wavelength Division Multiplexing) optical network is connected to LANs (Local Area Networks), metro (metropolitan) networks, metro access networks, and access networks. Conversion of access networks to optical technology, which enables much faster services, is widely anticipated. In addition, Passive Optical Network (PON) systems are also being introduced, providing real-time communications of greater capacity, at a reasonable cost. In these systems, the adoption of single-fibre bi-directional communications methods reduces the number of fibres required, and by sharing network-sides and fibre transmission paths between a number of users, faster services can be achieved at the same low cost of metal cable systems. Early installation of optical access networks is eagerly awaited worldwide, and the ITU-T and IEEE are actively discussing standardization in this field. The ITU-T has proposed standards for ATM-PON (G983.1) and B-PON (Broadband-PON) (G983.3). G983.1 relates to networks with a transmission path rate of 1 Mb/s or 6 Mb/s. In G983.3, these signals are multiplexed with CATV or other video signals. This essay starts with a brief discussion of B-PON systems, and goes on to introduce the 1 Mb/s OLT/ONT optical transmission module developed by Oki, which is a key device in optical systems of this kind. We will also look briefly at burst signal reception, which is a core technology in our transmission modules for optical access networks. Core D Optical Network Backbone Metro Metro/Access LAN LAN ATM/GbE-PON Systems Fig. 1 Overview of optical communications network 76 OKI Technical Review April 00/Issue 190 Vol.69 No.
Special issue on devices Integrated ONT (user side device) Basic Band ONT Transceiver Optical transmission Optical receiving Video-ONT Signal modes Burst No. 1 1.31/1.49µm 1.µm 1.49/1.µm Burst No. Continuous Upstream : 1.31 µm burst PON Downstream : 1.49/1. mm continuous Burst No. 3 Basic Band OLT (network side device) 1.49/1.µm Basic Band OLT Transceiver 1.µm Wavelength range 1.31/1.49µm Upstream Downstream Optical transmission Optical receiving Video-OLT Optical transmission Video 1.31µm 1.49µm 1.µm λ Fig. Broadband PON system Broadband PON systems Fig. shows the structure of a Broadband PON. In PON systems, information is distributed from the network-side OLT to all subscribers after encryption to ensure security. The user-side ONT is controlled so that signals do not collide with each other, and it sends a burst-shaped signal like the signal modes illustrated in Fig. to the network-side. The problem here is that the transmission distance, and other factors, differ for each subscriber, so as shown in Fig., the information is transmitted by optical signals of different amplitudes. To achieve a transmission sequence of this kind in a single optical fibre, the signals are sent by wavelength division multiplexing. The 1.31 µm wavelength is used in the upstream direction, whilst the 1.49 µm band is used for downstream signals, as well as the 1. µm band for supplying video signals. Video signals from a CATV office are converted to an optical signal by a Video OLT, and multiplexed by in the transmission path. On the ONT side, these signals are split by and converted to electrical signals by a V-ONT, which enables the subscriber to view the video images. Performance of OLT/ONT optical transmission module for 1 Mb/s communications Oki is already manufacturing a 1 Mb/s-compatible OLT/ONT optical transmission module which is compliant with ITU-T standard G983.1. Table 1 lists the specifications of the1 Mb/s OLT optical transmission module, whilst Fig. 3 shows an optical output Photo 1 1 Mb/s ONT optical transmission module waveform, and Fig. 4 shows error rate characteristics. Similarly, Table lists the specifications of the 1 Mb/s ONT optical transmission module, and its optical output waveform and error rate characteristics are shown in Fig. and Fig. 6, respectively. All of the given data can be guaranteed with satisfactory margins with respect to the standard values. Oki s burst data receiving technology As stated previously, the most important, but most difficult, technological challenge in creating a PON system is ensuring that burst signals from ONTs (userside) of different levels are all received with good sensitivity and good quality. OKI Technical Review April 00/Issue 190 Vol.69 No. 77
Table 1 Specifications of 1 Mb/s OLT optical transmission module Parameter Operating wavelength Mask of the transmitter eye diagram at transmitter wavelength Unit Specifications Tx : 1480-180/Rx : 160-1360 ITU-T G.983 Downstream Mean launched power range Minimum extinction ratio Tolerance to the transmitter incident light power m -4 ~ + 10 more than -1 Launched optical power w/o input to the transmitter Maximum spectral width Side mode suppression ratio Jitter tranfer Jitter generation in 1.3kHz bandwidth at receiver wavelength Minimum sensitivity Minimum overload Consecutive identical digit immunity Jitter tolerance Tolerance to the reflected optical power m UIpp m m bit 1 (@-0) more than 30 less than 0-30 -8 more than 7-10 10-4 degrees 70 degrees 0 degrees 10-10 -6 10-7 10-8 10-9 10-10 10-11 10-1 -38-36 -34-3 -30-8 -6 Received power[m] Fig. 3 1 Mb/s OLT optical output waveform Fig. 4 Error rate characteristics 78 OKI Technical Review April 00/Issue 190 Vol.69 No.
Special issue on devices Table Specifications of 1 Mb/s ONT optical transmission module Parameter Operating wavelength Mask of the transmitter eye diagram at transmitter wavelength Mean launched power range Minimum extinction ratio Unit m Specifications Tx : 160-1360/Rx : 1480-180 ITU-T G.983 Upstream -10-4 ~ + Tolerance to the transmitter incident light power Launched optical power w/o input to the transmitter m more than -1 less than -43 Maximum spectral width Side mode suppression ratio Jitter transfer Jitter generation in 1.3kHz bandwidth at receiver wavelength Minimum sensitivity Minimum overload Consective identical digit immunity Jitter tolerance Tolerance to the reflected optical power UIpp m m bit.8 (@rms) 0.1 (fc=130khz) 0. less than 0-30 -8 more than 7 0.7UIp-p (f0=6.khz) 0.07UIp-p (ft=6khz) -10 10-4 10 - degrees 70 degrees 0 degrees 10-6 10-7 10-8 10-9 10-10 10-11 10-1 10-13 -41-40 -39-38 -37-36 -3 Received power[m] Fig. 1 Mb/s ONT optical output waveform Fig. 6 Error rate characteristics OKI Technical Review April 00/Issue 190 Vol.69 No. 79
When burst type signals are received in a conventional receiving, it is impossible to regenerate accurately a small burst signal which follows a large burst signal, as depicted in Fig.. However, the newly developed ATC (Automatic Threshold Control) enables faithful signal regeneration, even when there are differences in level between the burst signals. This ATC is an automatic threshold level control, which sets an optimal threshold level, automatically, in accordance with the changing signal amplitudes. Fig. 7 shows an illustration of the actual regeneration of a burst signal. We can see clearly that Optical input waveform Electrical output waveform both the large burst signal and the small burst signal are regenerated correctly. Video ONT for Broadband-PON Oki is also developing a Video ONT for Broadband-PON to receive video signals. As illustrated in Fig., video signals created by passing signals from a CATV office through a Video OLT are distributed to each subscriber home, where they are regenerated by a Video ONT. Our new Video ONT has completed prototype evaluation and achieved excellent results which easily meet the G983.3 (1 Mb/s) standards. The main features of this prototype are given in Table 3. In our future plans, we aim to offer an interface transceiver designed for the broadband age, with an integrated ONT which incorporates the Video ONT described above, an ONT optical transmission module, and a filter. Future Issues Fig. 7 ATC operating characteristics Future challenges in this field are expected to include: (1) Cost reduction of ONT modules () High-speed Table 3 Specifications of Video ONT Parameter User Interface Frequency range Modulation Scheme RF Output Power Characteristic Impedance CNR Unit MHz - µv Ω Specifications Analog channel : 4-0, Digirtal Channel : 0-70 Analog channel : VSB-AM, Digirtal Channel : 64QAM +7 ~ +8 7 (unbalance) 48 (Optical input : 0m, fc=189.mhz single carrier) Network Interface Optical connector Receiver wavelength Optical moduration method Maximum received power Minimum received power mm m m SC type 13-16 Direct optical power modulation +6 - DC Input Voltage Consumption Power Condition V W Analog : ±, +9, Digital : ± <4 Over Condition AGC method PILOT frequency Dimension MHz mm PILOT AGC 73.0 10x7x17 80 OKI Technical Review April 00/Issue 190 Vol.69 No.
Special issue on devices (3) High-sensitivity PLC (Planar Lightwave Circuit) technology which are optical components is adopted for reduced costs of ONT modules. By using a system known as passive aligent, PLC technology enables self-align coupling to optical fibres, which makes it highly suitable for low-cost high-volume production. As for high speed, by providing faster services, it is possible to improve transmission efficiency and reduce service costs. At Oki, we plan to upgrade from our current products operating at 1 Mb/s, to speeds of 6 Mb/s and 1. Gb/s. High sensitivity, on the other hand, means that the split ratio can be expanded in line with increased operating speeds, enabling common facilities to be shared amongst a greater number of users, and thereby reducing infrastructure costs. Conclusion This essay has looked at transmission modules for optical access networks, with a particular focus on modules for PON systems and V-ONT modules. We already have a 1 Mb/s-compatible product on the market, and have scheduled the successive release of 6 Mb/s and 1. Gb/s versions in the future. Development of an Integrated ONT is also under way. PC and Internet use is predicted to carry on expanding in the future, and the drive towards lower costs and higher operating speeds is likely to increase momentum. At Oki, we aim to meet these demands by working towards. Gb/s and 10 Gb/s optical burst transmission modules, based on burst signal transmission technology and high-speed analogue IC technology which we own. Authors Hiroshi Ishizaki: Optical Components Company, Hiroshi Okada: Optical Components Company, Takayuki Tanaka: Optical Components Company, Yoshinori Arai: Optical Components Company, OKI Technical Review April 00/Issue 190 Vol.69 No. 81